The present invention relates generally to a giant magnetoresistive (GMR) read sensor for use in a magnetic read head. In particular, the present invention relates to a current-perpendicular-to-plane (CPP) read sensor having an enhanced giant magnetoresistive response.
GMR read sensors are used in magnetic data storage systems to detect magnetically-encoded information stored on a magnetic data storage medium such as a magnetic disc. A time-dependent magnetic field from a magnetic medium directly modulates the resistivity of the GMR read sensor. A change in resistance of the GMR read sensor can be detected by passing a sense current through the GMR read sensor and measuring the voltage across the GMR read sensor. The resulting signal can be used to recover the encoded information from the magnetic medium.
A typical GMR read sensor configuration is the GMR spin valve, in which the GMR read sensor is a multi-layered structure formed of a nonmagnetic spacer layer positioned between a synthetic antiferromagnet (SAF) and a ferromagnetic free layer. The magnetization of the SAF is fixed, typically normal to an air bearing surface of the GMR read sensor, while the magnetization of the free layer rotates freelyin response to an external magnetic field. The SAF includes a reference layer and a pinned layer which are magnetically coupled by a coupling layer such that the magnetization direction of the reference layer is opposite to the magnetization of the pinned layer. The resistance of the GMR read sensor varies as a function of an angle formed between the magnetization direction of the free layer and the magnetization direction of the reference layer. This multi-layered spin valve configuration allows for a more pronounced magnetoresistive effect, i.e. greater sensitivity and higher total change in resistance, than is possible with anisotropic magnetoresistive (AMR) read sensors, which generally consist of a single ferromagnetic layer.
A pinning layer is typically exchange coupled to the pinned layer of the SAF to fix the magnetization of the pinned layer in a predetermined direction. The pinning layer is typically formed of an antiferromagnetic material. In antiferromagnetic materials, the magnetic moments of adjacent atoms point in opposite directions and, thus, there is no net magnetic moment in the material.
An underlayer is typically used to promote the texture of the layers (including the pinning layer) consequently grown on top of it. The underlayer is chosen such that its atomic structure, or arrangement, corresponds with a desired crystallographic direction.
A seed layer is typically used to enhance the grain growth of the layers (including the underlayer) consequently grown on top of it. In particular, the seed layer provides a desired grain structure and size.
One principal concern in the performance of GMR read sensors is the xcex94R (the maximum absolute change in resistance of the GMR read sensor), which directly affects the GMR ratio. The GMR ratio (the maximum absolute change in resistance of the GMR read sensor divided by the resistance of the GMR read sensor multiplied by 100%) determines the magnetoresistive effect of the GMR read sensor. Ultimately, a higher GMR ratio yields a GMR read sensor with a greater magnetoresistive effect which is capable of detecting information from a magnetic medium with a higher linear density of data.
A key determinant of the GMR ratio is the material used for the coupling layer in the SAF. The sense current that is passed through the GMR read sensor consists of majority spin electrons (spin is in the same direction of the magnetization) and minority spin electrons (spin is in the opposite direction of the magnetization). Majority spin electrons exhibit very little resistance and enhance the signal produced by the sense current, while minority spin electrons exhibit very high resistance and diminish the signal produced by the sense current. In current-in-plane (CIP) read sensors, the sense current is passed through in a direction parallel to the layers of the read sensor. In order to maximize the mean free path of the majority spin electrons and the signal produced by the sense current, the majority spin electrons should be confined to the reference layer, free layer, and the spacer layer. It is therefore desirable for the coupling layer in the SAF to reflect majority spin electrons back into the reference layer in order to prevent the majority spin electrons from passing through into the pinned layer and scattering as minority spin electrons. In CPP read sensors, however, the sense current is passed through in a direction perpendicular to the layers of the read sensor. The reflection of majority spin electrons at the reference layer/coupling layer interface acts to increase the resistance of the majority spin electrons, which has the effect of diminishing the signal produced by the sense current. It is therefore desirable for the coupling layer to allow majority spin electrons to pass through without any appreciable scattering in order to enhance the signal produced by the sense current, and ultimately increase the GMR ratio of the read sensor. It is important, however, to ensure that the magnetic coupling between the reference layer and the pinned layer is maintained in order for the read sensor to function properly.
The present invention addresses these and other needs, and offers other advantages over current devices.
The present invention is a giant magnetoresistive (GMR) stack configured to operate in a current-perpendicular-to-plane (CPP) mode. The GMR stack includes a ferromagnetic free layer, at least one synthetic antiferromagnet (SAF), at least one nonmagnetic spacer layer, and at least one antiferromagnetic pinning layer. The ferromagnetic free layer has a rotatable magnetic moment. The SAF includes a ferromagnetic reference layer having a fixed magnetic moment, a ferromagnetic pinned layer having a fixed magnetic moment, and a coupling layer positioned between the reference layer and the pinned layer, wherein the coupling layer is selected from the group consisting of Cu, Ag and CuAg. The nonmagnetic spacer layer is positioned between the free layer and the SAF. The antiferromagnetic pinning layer is positioned adjacent to the SAF.